
Epic Boon Feat (Prerequisite: Level 19+)
You gain the following benefits.
Ability Score Increase. Increase one ability score of your choice by 1, to a maximum of 30.
Peerless Aim. When you miss with an attack roll, you can hit instead. Once you use this benefit, you can’t use it again until the start of your next turn.

Does this allow ranged/melee spell attacks to always hit if taken on a caster?
Yep. As defined by the Rule Glossary, an attack roll is any D20 Test from an attack with a weapon, an Unarmed Strike, or a spell.
Rules Glossary - Player’s Handbook - Dungeons & Dragons - Sources - D&D Beyond
Great on a rogue who only gets 1 attack anyway. If you don't have disadvantage just guarantee a sneak attack!